This invention relates to means for preventing premature failure of lamps in a D.C. system, especially lamps located on a highway trailer. Such highway trailers commonly have a number of lamps thereon; as for example, service clearance lamps located at the corners of the trailer to indicate the trailer outline, dome lights to illuminate the trailer interior space, stop lights (brake lights) at the rear of the trailer, turn indicator lights, and tail indicator lights. Trailers used by the military also have shielded blackout lights that are used to define the trailer outline dimensions when wartime conditions preclude use of the normal lighting system.
Commonly, a system can include as many as thirty lamps supplied from a voltage source in the tractor used to tow the trailer. A multiwire cable extends from the rear end of the tractor to an electrical receptacle mounted in the forward end of the trailer. Individual wires extend from terminals in the receptacle to individual lamps or groups of lamps on the trailer. In the event that one lamp should burn out, the available wattage is applied to a lesser number of lamps, thereby increasing the amperage through and voltage across each lamp in the associated system. The higher amperage and voltage tend to promote early failure of the remaining lamps, with a progressively increasing detrimental effect as each lamp burns out.
In the case of military applications, lighting system problems are somewhat complicated because the military services have adopted a policy of purchasing commercial tractors and trailers for reasons of lower cost and greater availability; the commercial units have 12 volt lighting systems instead of the 24 volts used on military equipment. When a military tractor having a 24 volt electrical system is used to haul a commercial trailer having a 12 volt lighting system the lamp burn-out problem becomes more acute. A solution to the lamp burn-out problem should take into account the fact that a 12 volt commercial tractor might be required to haul a 12 volt trailer or a 24 volt trailer; and a 24 volt military tractor might be required to haul a 12 volt trailer or a 24 volt trailer.
It has been proposed to provide resistors in the trailer electrical systems, arranged so that individual resistors are in series with individual lamps. However, in the case of commercial trailers, the resistors must be spliced in at the trailer electrical control box so that they can be bypassed, should the trailers be used with tractors having 12 volt systems. The use of resistors makes for a relatively complex and costly wiring harness.
The present invention is directed to the use of zener diodes for limiting the voltage or amperage applied to the individual lamps on a trailer, thereby preventing excessive current flows through the lamps as would tend to cause premature lamp failure. While it is technically possible to use a single large power zener diode in parallel with the entire lamp circuit we prefer to use a multiplicity of smaller zener diodes in parallel with individual lamps. When a large number of small zener diodes is used there is no requirement for a special zener diode cooling system, whereas a large power zener diode requires a special heat sink or other cooling mechanism.